Conventionally, porous beads are often used as the form of the adsorbent material to be used for the purification column which removes, by adsorption, a removal-object substance in the fluid that is to be treated. Examples of the reasons for this include the fact that bead-shaped adsorbent has advantages of less unbalanced blood flow in a column and hence ease of a column design because the adsorbent can be uniformly packed into an adsorption column. On the other hand, examples of a means for improving adsorption performance include increasing of a surface area per volume of the adsorbent. However, when the adsorbent is bead-like, a bead diameter is decreased in order to increase a surface area per volume of the adsorbent, a gap between the beads becomes narrow. Therefore, since resistance of a flow path becomes high to increase a pressure loss, it becomes difficult to pass the fluid that is to be treated. Further, the bead used as an adsorbent is usually spherical, and therefore it has a disadvantage that the surface area per volume is inherently small. That is, even though there is an adsorption reserve capacity inside the bead, an internal adsorption site cannot be effectively used.
Examples of the form of an adsorbent material other than the bead include a fiber, it is also thought to use a fiber having a common circular cross section. Examples of the form of the fiber include one obtained by inserting a large number of fibers in the straight form in parallel to a lengthwise direction of a column case, or one obtained by forming a knitting fabric.
In the knitting fabric of these forms, it is difficult in production to make the fiber porous for providing adsorption holes for the fiber. Further, when the fluid that is to be treated includes many dissolved substances and viscosity is high, it is not so preferred since processing easily results in a pressure rise in a column.
On the other hand, the fiber of the form obtained by inserting filament fibers such as a solid-state fiber or a hollow fiber in the straight form in parallel to a lengthwise direction of a column case, can secure a flow path of the fluid that is to be treated separately from the adsorbent material. Therefore, the fiber of this form can suppress resistance of the flow path, and is advantageous to the adhesion of a dissolved substance in the fluid that is to be treated. Hitherto, the inventions concerning a purification column into which hollow fibers or solid-state fibers are incorporated are disclosed (Patent Documents 1 and 2). However, a cross-section shape of the fibers used in these documents is circular, and the adsorption performance has been low since the surface area per volume of the adsorbent is small.
Here, a method in which a shape other than a circular shape is used as a cross-section shape of the fiber, that is, a fiber with a modified cross section is used, is known. However, it is heretofore thought that since spinning stability deteriorates when the modification degree of the fiber is increased, an increase of the modification degree is suppressed. Particularly, in the case of the porous fibers, there has been a fear that strength-elongation of the fiber is significantly reduced and unevenness of a fiber diameter referred to as draw resonance increases by a modified cross section, and in addition to this, there has been a fear that modification of a cross-section shape, particularly, agglutination of the projected part within a single fiber cross section, occurs.
However, heretofore, Patent Documents 3 to 5 describe the invention concerning the fibers with the modified cross section in which a shape other than a circular shape is used as a cross-section shape of the porous fiber. However, these fibers differ from aspects of the present invention in that all of the fibers pertain to a separation membrane of a hollow fiber type. In the case of the hollow fiber, modification of a cross-section shape described above hardly occurs since fixation of a structure can be performed at the same time from both side of the inside of a fiber (hollow portion) and the outside of a fiber in forming a fiber (=spinning). The fixation of a structure is performed through cooling by cool wind or through contact with a poor (non) solvent. Therefore, the hollow fiber is more advantageous than the solid-state fiber which can be cooled only from the outside of the fiber. As results of viewing each of concepts/objects of modifying a fiber shape in Patent Documents described above, prevention of intimate contact between bundles in bundling the fibers (Patent Document 3) and suppressing fouling by complicating and disturbing a flow on an outer surface of a hollow fiber ((Patent Documents 4 and 5) are mainly described. That is, shapes provided with short protrusions on the periphery of a fiber are merely employed for the object different from the present invention. Particularly, the above concept of suppressing fouling is contrary to the concept of an adsorption column which adsorbs a dissolved substance on the fiber. Accordingly, a concept of improving the adsorption performance by increasing a surface area per volume is not present. Therefore, a shape in which the modification degree is less-than-relatively high is shown.
In Patent Document 6, a separation membrane in which a modified cross section is formed is described. However, with respect to its “separation” function, a paragraph [0005] in the specification describes “one of performance indexes as a multilayer composite separation membrane is a permeation rate, and when a membrane material is the same, it is important to decrease a thickness of a separation layer and to increase a membrane area of a separation layer”. That is, it is assumed that an object-substance is separated by permeating a membrane. From such a viewpoint, a cross-section shape is modified aiming at an improvement of separation performance by increasing a membrane area of a separation membrane. Accordingly, specifically, a hollow fiber membrane is described, and it cannot be substantially said that a fiber of solid-state form is described.
That is, a technology of modifying a cross-section shape of a hollow fiber which is used in the above document is not a technology designed in consideration of use of a fiber as an adsorbent material.
On the other hand, Patent Document 7 describes the invention concerning the fibers with the modified cross section which has, but not having a hollow portion, pores on the surface. However, in aspects of this invention, pores are provided for splitting a fiber so as to increase a fiber volume and hence small. That is, the pore is largely different in pore size, pore size distribution and pore specific surface area from the pore for adsorption, and it is hard to say that the fiber is a material generally referred to as a porous fiber.